1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly, to a dynamic random access memory (DRAM) device with sense amplifiers as cache memories.
2. Description of the Related Art
Generally, in a low speed, large capacity main storage unit formed by DRAMs, in order to substantially increase access speed, a high speed, small capacity cache memory formed by bipolar transistors is incorporated into the main storage unit or provided externally for the main storage unit. However, since such a cache memory is very expensive, various cache substitution means have been suggested.
A prior art DRAM device uses sense amplifiers as a cache substitution means. That is, generally, if a memory section is divided into a plurality of memory cell blocks, each of the blocks is associated with a sense amplifier array which also has a storage function for the last accessed data. Therefore, the sense amplifier array is a cache substitution means, and can serve as a cache memory.
In order to make the sense amplifier arrays serve as cache memories, registers are provided for storing row addresses which were used for accessing the memory cell blocks, because the sense amplifier arrays store data finally accessed. This will be explained later in detail.
In the above-described DRAM device, however, if a self-refresh mode operation is carried out for automatically refreshing all the memory cells to maintain the data therein, all the cache data stored in the sense amplifier arrays are erased, thus reducing the performance of the cache memories.
Note that, if data of one of the memory cell blocks is read by using data of one of the registers and is restored in the corresponding sense amplifier array every time one refresh operation of the self-refresh mode is completed, the power dissipation is increased. Further, if data of all the memory cell blocks are read by using data of all the registers and are restored in all the sense amplifier arrays every time the self-refresh mode is completed, a time required for ending the refresh mode is increased.